1. Field of the Invention
This invention relates to drive methods of an array-type light modulation element manufactured by micromachining for changing the transmission factor of light by the electromechanical operation and a flat-panel display using the array-type light modulation element and in particular to alternating current (AC) drive of the array-type light modulation element and the flat-panel display.
2. Description of the Related Art
An electromechanical light modulation element for executing light modulation by causing a flexible thin film manufactured by micromachining to perform the mechanical operation by an electrostatic force is known. For example, the light modulation element comprises a flexible thin film made up of a transparent electrode and a diaphragm placed on a fixed electrode on a light waveguide plate via a support.
In the light modulation element, a predetermined voltage is applied across both electrodes, thereby generating an electrostatic force between the electrodes for bending the flexible thin film toward the fixed electrode. The optical characteristic of the element itself changes accordingly and light passes through the light modulation element. On the other hand, the applied voltage is set to zero, whereby the flexible thin film is elastically restored and the light modulation element shields light. Light modulation is thus executed.
By the way, to deform or elastically restore the light modulation element by an electrostatic force, the relationship between applied voltage Vgs and displacement of the flexible thin film shows a hysteresis characteristic. Therefore, the relationship between the applied voltage Vgs and light transmission factor T also shows a hysteresis characteristic as shown in FIG. 30.
According to the hysteresis characteristic, when the light modulation element is in an OFF state (fight shield), it holds the OFF state if Vgs is Vth(L) or less, and if Vgs becomes Vth(H) or more, the light modulation element holds an ON state. When Vgs is Vth(H) or more, the light modulation element remains in the ON state, and if Vgs becomes Vs(L) or less, the light modulation element becomes saturated to the OFF state. If the polarity of Vgs is negative, the characteristic symmetrical with respect to the vertical axis of the positive polarity results.
However, in the example in the related art, using the characteristic, the light modulation elements are placed in two dimensions and matrix drive is executed, whereby two-dimensional light modulation elements can be formed. As the matrix drive, simple matrix drive and active matrix drive are available and in either drive method, from the above-described modulation element, the matrix can be driven with one polarity of the voltage applied to the modulation element and a DC voltage component is applied to the modulation element.
However, in drive of applying the DC component, if the inside of the flexible thin film is formed of an insulator, there is a possibility that in the structure of electrode-insulator-electrode structure, a parasitic internal electric field, etc., may occur because of an insulation failure caused by migration of the insulator or occurrence/trap of spatial charges, causing the operation to become unstable. For liquid crystal, etc., an AC-drive system is adopted to avoid instability of drive of applying the DC component; particularly, to make the insulator of macromolecule, etc., the possibility that an unstable state will be entered is large.